Sister species within the Triops cancriformis lineage (Crustacea, Notostraca)

We investigated the phylogenetic relationships among the three presently recognized subspecies of the tadpole shrimp, Triops cancriformis , using mitochondrial 16S and 12S rDNA sequences. Our results indicate that the taxon is divided into two distinct lineages. One lineage is formed of T. c. cancriformis populations and samples from northern Spain that had been classified as T. c. simplex in the most recent literature. The second lineage comprises all populations of T. c. mauritanicus and northern African populations of T. c. simplex . These two main lineages separated 2.3 to 8.9 million years ago, based on the range of inferred molecular clocks recognized for crustacean mtDNA sequence divergence. Percentages of divergence are in the range reported for recognized species in other notostracan lineages and we therefore propose to recognize them as two species, Triops cancriformis and Triops mauritanicus . The latter would comprise two subspecies in northern Africa, one consisting of the Moroccan populations of the former T. c. mauritanicus , the other comprising the African populations of the former T. c. simplex . It also includes three as-yet unnamed lineages. A comparison of morphological characters with the molecular data revealed that the former T. c. simplex cannot be reliably separated from T. c. cancriformis , using morphological characters that have hitherto been used to distinguish among subspecies of T. cancriformis . Our investigation is the first to demonstrate the presence of T. c. cancriformis in Africa (Tunisia). The genetic haplotypes of these populations are identical with haplotypes also occurring in Central and Western Europe, as well as in Sicily. Therefore, we hypothesize that the African populations of T. c. cancriformis represent a result of repeated long-distance dispersal across the Mediterranean Sea.     

Reproductive isolation and genetic differentiation in North American species of Triops (Crustacea: Branchiopoda: Notostraca)

Electrophoretic analysis of 31 populations, ranging from California to Kansas, indicates that the North American tadpole shrimp Triops longicaudatus (LeConte) is actually a mixture of at least two reproductively isolated species. In the central United States, the predominant species is T. longicaudatus, which is found typically in ephemeral prairie pools. In the southwestern United States, the predominant species is Triops newberryi (Packard), which characteristically inhabits large playa pools. The two species coexist occasionally and in sympatric situations they are reproductively isolated from each other. The two species are genetically distinct at a level greater than is typical of conspecific populations. These genetic differences are correlated with subtle morphological differences and profound differences in reproductive biology. Both species represent complexes of bisexual and unisexual populations. Bisexual populations of T. longicaudatus are usually composed of males and females in approximately equal frequencies; bisexual populations of T. newberryi are composed of males in low frequencies and self-compatible hermaphrodites. Unisexual populations of both species apparently consist entirely of self-compatible hermaphrodites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Molecular taxonomy and phylogeny of the 'living fossil' lineages Triops and Lepidurus (Branchiopoda: Notostraca)

Mantovani B. Cesari M. & Scanabissi F. (2004). Molecular taxonomy and phylogeny of the 'living fossil' lineages Triops and Lepidurus (Branchiopoda: Notostraca). — Zoologica Scripta , 33 , 367–374.
European Triops cancriformis and Lepidurus apus were analysed for 12S and 16S mitochondrial genes and compared to North American and Japanese taxa. There are no cryptic species among European T. cancriformis populations , which are highly homogeneous in comparison to conspecific Japanese samples. T. cancriformis differs from congeneric taxa all over its range, which can be explained by its antiquity. In contrast, the parapatric subspecies L. apus apus and L. apus lubbocki are morphologically conserved and differ substantially at the mtDNA level. The genetic distance values between them are of the same order of magnitude as those observed between American Lepidurus species. Their subspecific status therefore requires further analysis. L. apus apus is more closely related to a L. arcticus sample from Iceland than to L. apus lubbocki . It is also related to a Canadian L. couesii population. Further analyses of populations from the whole range of L. arcticus and the European range of L. couesii are needed to understand the relationships among these notostracan taxa. When considering the two genera, it is clear that Lepidurus is a well supported monophyletic unit, while Triops is polyphyletic, embodying very divergent taxa.     

Molecular characterization of visual pigments in Branchiopoda and the evolution of opsins in Arthropoda

Studies on color vision in invertebrates have focused primarily on insect visual pigments, with little attention given to crustacean visual pigments. None of the blue-green-, blue-, or ultraviolet (UV)-sensitive-opsins have been identified in crustaceans. In addition, the discussion of visual pigments has been limited to long-wavelength-sensitive opsins in Pancrustacea. Here, we focused on Branchiopoda (Crustacea), which is a sister group of Hexapoda including insects. In the tadpole shrimp Triops granarius, the visual pigment chromophore was retinal. Multiple opsins were isolated from each of three branchiopod species, T. granarius, Triops longicaudatus, and the fairy shrimp Branchinella kugenumaensis (five, five, and four opsins from these species, respectively). Phylogenetic analyses and the presence of a lysine residue corresponding to position 90 in bovine rhodopsin suggested that three of the branchiopod opsins comprise UV-sensitive pigments. In addition, the phylogenetic relationships between insect and branchiopod UV-sensitive opsins revealed that the divergence of blue- and UV-sensitive pigments predates the Branchiopoda and Insecta divergence. The other branchiopod opsins show distant relationships to other known insect opsins and form novel clusters. The present results strongly suggest that the ancestral arthropod of the Chelicerata-Pancrustacea lineages possessed at least four types of opsins. The ancestors of Pancrustacea and the Insecta-Branchiopoda lineages possessed at least five and six types of opsins, respectively. Our results suggest that in the evolutionary process associated with each lineage, several opsins appeared and diversified with repeated gene duplication, of which some have been lost in some taxa.     

Consequences of tadpole shrimp predation on mayflies in some Californian ponds

SUMMARY. 1. During two, 1-month studies, the abundance of mayfly nymphs, Callibaetis californicus Banks, in small ponds that contained the tadpole shrimp, Triops longicaudatus (LeConte), often was significantly lower than in ponds without tadpole shrimps. Mayfly abundance was either positively correlated or not significantly associated with that of predacious insects.
2. Larger mayfly nymphs were a greater fraction of the population in ponds with Triops than in ponds without tadpole shrimps. Between 2 and 3 weeks after ponds were flooded, the smallest mayfly nymphs were absent from ponds in which T. longicaudatus densities were >5 individuals per square metre during the summer and >15 individuals per square metre during the autumn.
3. In prey-choice trials, T. longicaudatus consumed predominantly small C. californicus nymphs (mean headwidth ± 1 SD: 0.44 ± 0.05 mm). After encounters with tadpole shrimps, small mayfly nymphs moved relatively shorl distances, were easily overtaken, and were, consequently, more susceptible to tadpole shrimps than larger-sized nymphs.     

Sex ratio, reproductive mode and genetic diversity in Triops cancriformis

1. Aquatic invertebrates display a wide array of alternative reproductive modes from apomixis to hermaphroditism and cyclical parthenogenesis. These have important effects on genetic diversity and population structure. Populations of the 'living fossil' Triops cancriformis display a range of sex ratios, and various reproductive modes are thought to underlie this variation. Using sex ratio information and histological analyses European populations have been inferred to be gonochoric (with separate males and females), selfing hermaphroditic and androdioecious, a rare reproductive mode in which selfing hermaphrodites coexist with variable proportions of males. In addition, some populations have been described as meiotic parthenogens.
2. Here we use population genetic analysis using microsatellite loci in populations with a range of sex ratios including a gonochoric population, and marker segregation patterns in heterozygote individuals reared in isolation, to clarify the reproductive mode in this species.
3. Our data show that populations in general have very low levels of genetic diversity. Non-gonochoric populations show lower genetic diversity, more heterozygote deficiencies, higher inbreeding coefficients and stronger linkage disequilibria than the gonochoric population. The maintenance of some heterozygosity in populations is consistent with some male influence in T. cancriformis populations, as would be expected from an androdioecious reproductive system. Results of marker segregation in eggs produced in isolation from non-gonochoric populations indicate that meiosis occurs and are consistent with two reproductive modes: selfing hermaphroditism and a type of ameiotic parthenogenesis.
4. Overall, our data indicate that androdioecy and selfing hermaphroditism are the most likely reproductive modes of non-gonochoric European Triops populations. Triops populations are strongly structured, suggesting high genetic drift and low levels of gene flow.     

Trehalose and vitreous states: desiccation tolerance of dormant stages of the crustaceans Triops and Daphnia

Several aquatic organisms are able to withstand extreme desiccation in at least one of their life stages. This is commonly known as "anhydrobiosis." It was often thought that to tolerate such a desiccated state required high amounts of compatible solutes such as the nonreducing disaccharide trehalose, which protects cellular structures by water replacement and glass formation. Trehalose levels of dormant eggs and cysts of five freshwater crustaceans (Daphnia magna, Daphnia pulex, Triops longicaudatus, Triops cancriformis, and Triops australiensis) were observed in different states of hydration and dehydration. Although trehalose was detected in all species, the concentration was under 0.5% of the dry weight (0.05 μg/μg protein), and no change between the different states was observed. Differential scanning calorimetry (DSC) measurements indicated that dried cysts of all Triops species were in a glassy state, supporting the vitrification hypothesis. No indication for a vitreous state was found in dried resting eggs of Daphnia.     

Additional data for nuclear DNA give new insights into the phylogenetic position of Sorex granarius within the Sorex araneus group

Many species contain genetic lineages that are phylogenetically intermixed with those of other species. In the Sorex araneus group, previous results based on mtDNA and Y chromosome sequence data showed an incongruent position of Sorex granarius within this group. In this study, we explored the relationship between species within the S. araneus group, aiming to resolve the particular position of S. granarius. In this context, we sequenced a total of 2447 base pairs (bp) of X-linked and nuclear genes from 47 individuals of the S. araneus group. The same taxa were also analyzed within a Bayesian framework with nine autosomal microsatellites. These analyses revealed that all markers apart from mtDNA showed similar patterns, suggesting that the problematic position of S. granarius is best explained by an incongruent behavior by mtDNA. Given their close phylogenetic relationship and their close geographic distribution, the most likely explanation for this pattern is past mtDNA introgression from S. araneus race Carlit to S. granarius.     

Sexual dimorphism and intraspecific variation in wing size and shape of Tongeia fischeri (Lepidoptera: Lycaenidae)

Wing morphological variations are described here for the lycaenid butterfly Tongeia fischeri. A landmark‐based geometric morphometric approach based on wing venation of 197 male and 187 female butterflies collected in Japan was used to quantify wing size and shape variations between sexes and among populations. Sexual dimorphism in wing size and shape was detected. Females had significantly larger wings than males, while males showed a relatively elongated forewing with a longer apex and narrower wing tornus in comparison to females. Intraspecific variations in wing morphology among populations were revealed for the wing shape, but not wing size. Distinct wing shape differences were found in the vein intersections area around the distal part of the discal cell where median veins originated in the forewing and around the origin of the CU1 vein in the hindwing. In addition, phenotypic relationships inferred from wing shape variations grouped T. fischeri populations into three groups, reflecting the subspecies classification of the species. The spatial variability and phenotypic relationships between conspecific populations of T. fischeri detected here are generally in agreement with the previous molecular study based on mitochondrial and nuclear sequences, suggesting the presence of a phylogenetic signal in the wing shape of T. fischeri, and thus having taxonomic implications.     

ACCELERATED EVOLUTION OF LAND SNAILS MANDARINA IN THE OCEANIC BONIN ISLANDS: EVIDENCE FROM MITOCHONDRIAL DNA SEQUENCES

An endemic land snail genus Mandarina of the oceanic Bonin (Ogasawara) Islands shows exceptionally rapid evolution not only of morphological and ecological traits, but of DNA sequence. A phylogenetic relationship based on mitochondrial DNA (mtDNA) sequences suggests that morphological differences equivalent to the differences between families were produced between Mandarina and its ancestor during the Pleistocene. The inferred phylogeny shows that species with similar morphologies and life habitats appeared repeatedly and independently in different lineages and islands at different times. Sequential adaptive radiations occurred in different islands of the Bonin Islands and species occupying arboreal, semiarboreal, and terrestrial habitat arose independently in each island. Because of a close relationship between shell morphology and life habitat, independent evolution of the same life habitat in different islands created species possesing the same shell morphology in different islands and lineages. This rapid evolution produced some incongruences between phylogenetic relationship and species taxonomy. Levels of sequence divergence of mtDNA among the species of Mandarina is extremely high. The maximum level of sequence divergence at 16S and 12S ribosomal RNA sequence within Mandarina are 18.7% and 17.7%, respectively, and this suggests that evolution of mtDNA of Mandarina is extremely rapid, more than 20 times faster than the standard rate in other animals. The present examination reveals that evolution of morphological and ecological traits occurs at extremely high rates in the time of adaptive radiation, especially in fragmented environments.     

Toward a global phylogeny of the "living fossil" crustacean order of the Notostraca

Tadpole shrimp (Crustacea, Notostraca) are iconic inhabitants of temporary aquatic habitats worldwide. Often cited as prime examples of evolutionary stasis, surviving representatives closely resemble fossils older than 200 mya, suggestive of an ancient origin. Despite significant interest in the group as 'living fossils' the taxonomy of surviving taxa is still under debate and both the phylogenetic relationships among different lineages and the timing of diversification remain unclear. We constructed a molecular phylogeny of the Notostraca using model based phylogenetic methods. Our analyses supported the monophyly of the two genera Triops and Lepidurus, although for Triops support was weak. Results also revealed high levels of cryptic diversity as well as a peculiar biogeographic link between Australia and North America presumably mediated by historic long distance dispersal. We concluded that, although some present day tadpole shrimp species closely resemble fossil specimens as old as 250 mya, no molecular support was found for an ancient (pre) Mesozoic radiation. Instead, living tadpole shrimp are most likely the result of a relatively recent radiation in the Cenozoic era and close resemblances between recent and fossil taxa are probably the result of the highly conserved general morphology in this group and of homoplasy.     

A new perspective on the evolutionary history of western European Sorex araneus group revealed by paternal and maternal molecular markers

The species of the common shrew (Sorex araneus) group are morphologically very similar, but have undergone a spectacular chromosomal evolution. We investigate here the evolutionary history of the Sorex araneus group distributed in western Europe. In particular, we clarify the position of a difficult species, S. granarius, using sex-specific (mtDNA and Y-chromosome) markers. The karyotype of S. granarius is generally considered similar to the common ancestor of the restricted group considered here. The mtDNA data (1.4 kb) confirms the close relationship between S. granarius and S. araneus sensu stricto (hereafter S. araneus s.s.), but the Y-chromosome (3.4 kb) produces a quite different picture: S. granarius is closely related to another species, S. coronatus. Comparison of mtDNA and Y-chromosome phylogenies suggests that the genetic and chromosomal evolution in this group are disconnected processes. The evolutionary history of the south-western European populations of the S. araneus group can only be understood considering secondary contacts between taxa after their divergence, implying genetic exchanges by means of hybridization and/or introgression.     

Phylogeny,molecular ecology and taxonomy of southern Iberian lineages of <Emphasis Type="Italic">Triops mauritanicus</Emphasis> (Crustacea: Notostraca)

We investigated the phylogeography of the main lineages in the tadpole shrimp Triops mauritanicus Ghigi in the south-western Iberian Peninsula, using mitochondrial 12S and 16S rDNA sequences. Our results indicate that a fourth, hitherto unknown main phylogenetic lineage occurs in Iberia, so that in total, the species is divided into six distinct clades, comprising T. m. mauritanicus, T. m. simplex Ghigi, and four as yet unnamed lineages that appear to be endemic to Iberia. Percentages of sequence divergence among the main clades in T. mauritanicus reach the range reported for recognized species in other notostracan lineages. A thorough morphological investigation also revealed that the differentiation among these lineages is higher than previously thought, and that populations of three of the main clades within T. mauritanicus can be reliably separated from each other and from the remaining lineages based on the morphology of adult males. The remaining clades also show a significant level of morphological differentiation, but include a certain proportion of populations for which the additional application of molecular methods is needed for a reliable determination. The geographic distributions of 12S haplotypes are indicative of frequent dispersal events and gene flow among populations belonging to the same main lineage, but give no evidence of recent migration events among different main lineages, suggesting that there is no gene flow among the latter. Our data thus suggest that the six main lineages within T. mauritanicus represent distinct species. We therefore describe the Iberian lineages as T. baeticus Korn n. sp., T. emeritensis Korn & Pérez-Bote n. sp., T. gadensis Korn & García-de-Lomas n. sp., and T. vicentinus Korn, Machado, Cristo & Cancela da Fonseca n. sp., and reinstate T. simplex Ghigi to full species status. Our data confirm the general, previously recognized pattern of a lower dispersal probability in gonochoric Triops taxa. However, we found evidence that passive dispersal in Triops may be further complicated by a strong habitat dependence of dispersal probability, mediated by prevailing dispersal vectors.     

Phylogenetic relationships between tuna species of the genus Thunnus (Scombridae: Teleostei): Inconsistent implications from morphology,nuclear and mitochondrial genomes

In order to infer phylogenetic relationships between tuna species of the genus Thunnus, partial sequences of the mitochondrial cytochrome b and ATPase genes were determined in all eight species. Supplemental restriction analysis on the nuclear rRNA gene was also carried out. Pacific northern bluefin tuna (Thunnus thynnus orientalis) was found to have mtDNA distinct from that of the Atlantic subspecies (T. t. thynnus) but very similar to that from the species albacore (T. alaluga). In contrast, no differentiation in nuclear genome was observed between the Atlantic and Pacific northern bluefin tunas. The Atlantic northern bluefin and southern bluefin tunas possessed mtDNA sequences very similar to species of yellowfin tuna group and not so similar to albacore and bigeye tunas which were morphologically assigned to the bluefin tuna group. The molecular data indicate that (1) mtDNA from albacore has been incorporated into the Pacific population of northern bluefin tuna and has extensively displaced the original mtDNA, and (2) albacore is the earliest offshoot, followed by bigeye tuna in this genus, which is inconsistent with the phylogenetic relationships between these tuna species inferred from morphology. Correspondence to: S. Chow     

Neurogenesis in the developing visual system of the branchiopod crustacean Triops longicaudatus (LeConte, 1846): corresponding patterns of compound-eye formation in Crustacea and Insecta?

In the discussion on arthropod phylogeny, the structural evolution of compound eyes and optic ganglia in Crustacea and Insecta is an important topic. On the one hand, many morphological features as well as developmental aspects of the visual system in Insecta and Crustacea correspond in so much detail that eye design in these two groups is likely to have a common euarthropodan ancestor. On the other hand, however, some authors advocate a convergent evolution of the crustacean and insect visual system founding their arguments on differences in the arrangement of the visual neuropils and the fibre connections between Malacostraca and Entomostraca (the "entomostracan enigma"). Therefore, information about cellular aspects of visual system formation in entomostracan Crustacea is likely to enliven this debate, but is not yet available. To fill this gap, we examined the proliferation of neuronal stem cells in the developing visual system of the tadpole shrimp Triops longicaudatus (LeConte, 1846) (Entomostraca, Branchiopoda, Phyllopoda, Calmanostraca, Notostraca) by in vivo incorporation of the proliferation marker bromodeoxyuridine and subsequent immunohistochemical detection. Our results indicate that in the developing visual system of T. longicaudatus, three band-shaped zones containing neuronal stem cells are present corresponding to the proliferation zones found in Malacostraca. We therefore conclude that the ontogenetic mechanisms of visual-system formation are evolutionarily conserved (homologous) in Branchiopoda, Malacostraca, and Insecta.     

Recent evolution of alternative reproductive modes in the 'living fossil' <Emphasis Type="Italic">Triops cancriformis</Emphasis>

Background  

The Notostraca is a small but ancient crustacean order with a contrasting combination of a conservative morphology and a wide range of reproductive modes. The tadpole shrimp Triops cancriformis, includes bisexual – the putatively ancestral state -, androdioecious and hermaphrodite populations. As hermaphroditism and androdioecy confer a colonisation advantage, we expect the postglacial colonisation of northern Europe to have been effected by lineages with such reproductive modes. Therefore, N European populations should be composed of closely related lineages reflecting a recent range expansion. In contrast, glacial refugia in the south should contain bisexual populations with high haplotype diversity and more population structuring. To test these hypotheses, we analysed the geographic distribution of reproductive modes based on new and published sex ratio data. In addition, we investigated the European phylogeography of T. cancriformis by sequencing over a 1000 bp of mitochondrial DNA (mtDNA) in individuals from a large sample of populations of the three recognised subspecies.     

The geographic structure of morphological variation in eight species of fiddler crabs (Ocypodidae: genus Uca) from the eastern United States and Mexico

Species with larger geographic distributions are more likely to encounter a greater variety of environmental conditions and barriers to gene flow than geographically‐restricted species. Thus, even closely‐related species with similar life‐history strategies might vary in degree and geographic structure of variation if they differ in geographic range size. In the present study, we investigated this using samples collected across the geographic ranges of eight species of fiddler crabs (Crustacea: Uca) from the Atlantic and Gulf coasts of North America. Morphological variation in the carapace was assessed using geometric morphometric analysis of 945 specimens. Although the eight Uca species exhibit different degrees of intraspecific variation, widespread species do not necessarily exhibit more intraspecific or geographic variation in carapace morphology. Instead, species with more intraspecific variation show stronger morphological divergence among populations. This morphological divergence is partly a result of allometric growth coupled with differences in maximum body size among populations. On average, 10% of total within‐species variation is attributable to allometry. Possible drivers of the remaining morphological differences among populations include gene flow mediated by ocean currents and plastic responses to various environmental stimuli, with isolation‐by‐distance playing a less important role. The results obtained indicate that morphological divergence among populations can occur over shorter distances than expected based on dispersal potential. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 248–270.     

Invasion history and demographic processes associated with rapid morphological changes in the Red‐whiskered bulbul established on tropical islands

The Red‐whiskered bulbul is a very successful invasive bird species. Morphological differences have been reported among individuals inhabiting the humid and dry coasts of Reunion Island, in a 30‐year‐old population. This suggests a capacity for rapid local adaptation which could explain the general invasive success of this species. However, the origin and invasion history of this population is unknown. It is therefore not possible to establish with certainty the cause of these morphological differences. Here, we investigated the invasion history of populations of Red‐whiskered bulbul established on Reunion Island, Mauritius and Oahu (three geographically similar tropical islands) to assess the link between invasion history and morphological changes in these populations. We first assessed the source(s) of the invasive populations. We then compared the morphology of the individuals between the invasive and native populations and between the dry and humid coasts of invaded islands. Finally, we inferred the invasion history of the invasive populations to investigate the role of neutral processes (e.g. founder effect and drift) on morphology. We found that the invasive populations have a similar origin and that the morphology of the individuals in these populations has diverged in a similar way from the native range, suggesting a convergent adaptation to tropical islands. Like on Reunion, we found differences in morphology between the dry and humid coasts on Mauritius. These morphological differences can be explained by invasion history on Reunion but not on Mauritius. Both neutral evolution and adaptation thus shape the morphology of invasive Red‐whiskered bulbuls.